preliminary study on atmospheric- pressure plasma-based ......preliminary study on...

Preliminary study on atmospheric-pressure plasma-based chemical dryfiguring and finishing of reaction-sintered silicon carbide

Xinmin ShenHui DengXiaonan ZhangKang PengKazuya Yamamura

Xinmin Shen, Hui Deng, Xiaonan Zhang, Kang Peng, Kazuya Yamamura, “Preliminary study onatmospheric-pressure plasma-based chemical dry figuring and finishing of reaction-sintered siliconcarbide,” Opt. Eng. 55(10), 105102 (2016), doi: 10.1117/1.OE.55.10.105102.

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Preliminary study on atmospheric-pressure plasma-basedchemical dry figuring and finishing of reaction-sinteredsilicon carbide

Xinmin Shen,a,* Hui Deng,b Xiaonan Zhang,a Kang Peng,a and Kazuya Yamamurac,*aPLA University of Science and Technology, College of Field Engineering, Research Center for Mechanical and Electrical Engineering,Haifu Street, Nanjing, Jiangsu 210007, ChinabSingapore Institute of Manufacturing Technology, 71 Nanyang Drive, 638075, SingaporecOsaka University, Graduate School of Engineering, Research Center for Ultra-Precision Science and Technology, 2-1 Yamadaoka, Suita,Osaka, 565-0871, Japan

Abstract. Reaction-sintered silicon carbide (RS-SiC) is a research focus in the field of optical manufacturing.Atmospheric-pressure plasma-based chemical dry figuring and finishing, which consist of plasma chemicalvaporization machining (PCVM) and plasma-assisted polishing (PAP), were applied to improve material removalrate (MRR) in rapid figuring and ameliorate surface quality in fine finishing. Through observing the processed RS-SiC sample in PCVM by scanning white-light interferometer (SWLI), the calculated peak-MRR and volume-MRRwere 0.533 μm∕min and 2.78 × 10−3 mm3∕min, respectively. The comparisons of surface roughness and mor-phology of the RS-SiC samples before and after PCVM were obtained by the scanning electron microscope andatomic force microscope. It could be found that the processed RS-SiC surface was deteriorated with surfaceroughness rms 382.116 nm. The evaluations of surface quality of the processed RS-SiC sample in PAPcorresponding to different collocations of autorotation speed and revolution speed were obtained by SWLI meas-urement. The optimal surface roughness rms of the processed RS-SiC sample in PAP was 2.186 nm. Therewere no subsurface damages, scratches, or residual stresses on the processed sample in PAP. The resultsindicate that parameters in PAP should be strictly selected, and the optimal parameters can simultaneouslyobtain high MRR and smooth surface. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 UnportedLicense. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.OE.55.10.105102]

Keywords: optical machining; reaction-sintered silicon carbide; plasma chemical vaporization machining; plasma-assisted polishing;material removal rate; surface quality.

Paper 161174 received Jul. 31, 2016; accepted for publication Sep. 22, 2016; published online Oct. 18, 2016.

1 IntroductionReaction-sintered silicon carbide (RS-SiC) ceramic is apromising optical material because of its excellent mechani-cal, chemical, thermal, and electrical properties,1,2 whichmakes it a research focus in the field of optical machining.However, it is difficult to obtain a high material removal rate(MRR) in figuring RS-SiC for its high mechanical hardnessand strong chemical inertness.3,4 What is more, the fabrica-tion process of RS-SiC generates SiC and Si domains in theRS-SiC substrate, and the asymmetric components make itdifficult to obtain a high surface quality in the fine finishingof RS-SiC.5,6 Along with the increasing demands of RS-SiCproducts with high quality, it is urgently required to developunique processing techniques.

Many techniques have been developed to process thistraditional difficult-to-machine material, but few of them cansimultaneously obtain high MRR and fine surface quality inmachining RS-SiC.7 An efficient way is the combination ofa rapid figuring method and a fine finishing method. Plasmachemical vaporization machining (PCVM) is a machiningmethod with chemical material removal by the F radical.8,9

Owing to the controllable chemical reaction rate, the MRRin PCVM can be enlarged by adjusting the processing

parameters.10 Plasma-assisted polishing (PAP) includes theoxidation of the substrate by the water vapor plasma andthe removal of oxide by the abrasive polishing.11 Due to thehardness differences between the substrate and oxide, PAPcan achieve an ultrasmooth surface, which has been verifiedin machining single crystal 4H-SiC samples.12,13 Therefore,the application of atmospheric-pressure plasma-based chemi-cal dry figuring and finishing, which consist of PCVM andPAP, is investigated to verify its feasibility to process RS-SiC in this study.

After PCVM of RS-SiC samples, scanning white-lightinterferometer (SWLI) measurement was conducted to cal-culate the MRR and evaluate the surface quality, and thescanning electron microscope (SEM) and the atomic forcemicroscope (AFM) were used to investigate the surfacemorphology. Meanwhile, PAP was applied to improve thesurface quality of the processed RS-SiC sample in PCVM,and the surface roughnesses corresponding to differentpolishing parameters were obtained by SWLI measurements.By the investigation on MRR in PCVM and the study onsurface quality in PAP, the feasibility of these techniqueswas verified.

2 Experimental ApparatusThe schematic diagram of the PCVM system is shown inFig. 1(a).14,15 The electrode for generating plasma was con-structed from coaxially arranged metal electrode and alumina

*Address all correspondence to: Xinmin Shen, E-mail: [email protected]; Kazuya Yamamura, E-mail: [email protected]

Optical Engineering 105102-1 October 2016 • Vol. 55(10)

Optical Engineering 55(10), 105102 (October 2016)


http://dx.doi.org/10.1117/1.OE.55.10.105102

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http://dx.doi.org/10.1117/1.OE.55.10.105102

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(Al2O3) ceramic tube. The diameter and material of the elec-trode were 3 mm and aluminum (Al) alloy, respectively. Thecomposition and flow rate of the processing gas were con-trolled by the mass flow controller. Supplying of the gas fromthe circumference of the tip of the electrode replaced theair to the processing gas and enabled the stable generation ofthe plasma in the atmospheric-pressure environment. Theapplied high-frequency electric field (f ¼ 13.56 MHz) gen-erated the plasma in the narrow gap between the electrodeand the substrate, and the typical gap distance was 500 μm.

The schematic diagram of the PAP system is shown inFig. 1(b).16,17 The compound apparatus consisted of theinstalled plasma generation part and the mechanical polish-ing part. Atmospheric-pressure plasma was generated byapplying an RF (f ¼ 13.56 MHz) electric power, and helium(He)-based water vapor with a flow rate of 1.5 L∕min wassupplied as the process gas. Water vapor was introduced intothe process gas by bubbling the He through ultrapure water,and its concentration was measured by the dew-point meter.The used copper (Cu) electrode was covered with quartzglass to prevent the arc discharge in the generation of dielec-tric barrier discharge. In the mechanical polishing part, theRS-SiC sample was fixed on the rotary table. The resin

bonded ceria (CeO2) grinding stone was installed on the pol-ishing head. An insulative adapter was used to connect thespindle and the electrode, which could protect the motor. Theload was set to 10 g, which was propitious to obtain highsurface quality in the polishing process. First, the RS-SiCsample was processed by PCVM. Afterward, PAP was con-ducted to finish the surface of the processed RS-SiC samplein PCVM.

3 Results and Discussion

3.1 Reaction-Sintered Silicon Carbide SampleProcessed by Plasma Chemical VaporizationMachining

TheMRR of RS-SiC in PCVMwas obtained by SWLI meas-urement, as shown in Fig. 2. The RF power, compositionsand flow rates of the process gases, and machining timewere 100 W, He (750 ml∕min), CF4 (5 ml∕min), O2

(5 ml∕min), and 90 s, respectively. It could be calculatedthat the peak-MRR was 0.533 μm∕min, and the volume-MRR was 2.78 × 10−3 mm3∕min. Relative to the presentmethods for machining RS-SiC, such as the single-point dia-mond turning,18 electrolytic in-process dressing grinding,19

Fig. 1 Schematic diagram of the experimental apparatus: (a) PCVM and (b) PAP systems.

Fig. 2 Calculation of the MRR in PCVM of RS-SiC sample obtained by SWLI measurement: (a) typicalshape of the removal spot and (b) cross-sectional line of the removal spot.


Shen et al.: Preliminary study on atmospheric-pressure plasma-based chemical dry figuring and finishing. . .


and magnetorheological finishing,20 the MRR in PCVM ofRS-SiC is considerable. What is more, PCVM is a noncon-tact figuring technique and the material removal is conductedby chemical reaction, so we suppose that there will be nosubsurface damage or residual stresses on the processedsample, which are common in the processed RS-SiC sampleobtained by the present mechanical material removal meth-ods. Therefore, PCVM can be treated as an efficient rapidfiguring method to process RS-SiC.

The comparisons of surface roughness of the RS-SiCsample before and after PCVM are shown in Fig. 3. Itcan be found that the initial surface, which was obtainedby diamond lapping, had a fine surface roughness rms0.910 nm. However, from Fig. 3(a), it can be found thatthere were scratches on the surface, because the hardness ofthe diamond is higher than that of the RS-SiC. Furthermore,there were small holes on the surface as shown in Fig. 3(a),because the hardnesses of SiC and Si grains in the RS-SiCsubstrate differ, which resulted in a different micro-MRRamong the different grains. After PCVM, the RS-SiC surfacewas deteriorated, and the surface roughness root-mean-square

(rms) and roughness average (Ra) were 382.116 and242.427 nm, respectively. The major reason for this phe-nomenon might be that the chemical reactivities of SiCand Si grains differ. The possible chemical reactions thatoccurred in the PCVM of RS-SiC are as follows:

EQ-TARGET;temp:intralink-;e001;326;449SiCþ CF4 þ O2 → SiF4 þ 2CO; (1)

EQ-TARGET;temp:intralink-;e002;326;4172Siþ 2CF4 þ O2 → 2SiF4 þ 2CO: (2)

Therefore, the MRRs in machining SiC/Si grains by F rad-icals were nonuniform. The results obtained by SWLI indi-cated that the PCVM could obtain high MRR in processingRS-SiC but the obtained surface quality was rough. Thus,another method is urgently demanded to improve the surfacequality of the processed RS-SiC in PCVM, which can forman integrated technique.

The comparisons of the surface morphology of RS-SiCbefore and after PCVM at the same position were conductedby SEM, as shown in Fig. 4. From Fig. 4(a), it could be foundthat there were many small scratches and holes on the initial

Fig. 3 Comparisons of surface roughness of the RS-SiC sample before and after PCVM: (a) the initialsurface obtained by diamond lapping and (b) the processed surface obtained by PCVM.

Fig. 4 Comparisons of the surface morphology of RS-SiC sample at the same position obtained by SEM:(a) before and (b) after PCVM.




surface generated by diamond lapping, which was in accor-dance with the characters exhibited in Fig. 3(a). The surfaceof the processed RS-SiC sample in PCVM was bumpy, asshown in Fig. 4(b). Irregular holes were formed at the regionof initial Si grains, and the areas of initial SiC grains hadrelatively smooth surfaces. It could be concluded that themicro-MRR of Si grain was larger than that of SiC grainin the RS-SiC substrate. This phenomenon coincided withthe fact that the MRR in PCVM of single crystal Si wafer is

larger than that of single crystal SiC wafer under the sameexperimental parameters.21

The details of the processed RS-SiC sample in PCVM areshown in Fig. 5, which were obtained by SEM observationunder high magnification. The outlines of the newly exposedSiC grains were very clear in Fig. 5(a), especially for somelarge grains. Meanwhile, there were obvious boundariesamong the newly revealed Si grain and the SiC grains aroundit, as shown in Fig. 5(b), and the Si grain had the same

Fig. 5 Details of the processed RS-SiC sample in PCVM obtained by SEM observation under highmagnification: (a) the newly exposed SiC grains, (b) the newly revealed Si grains, (c) the shrunkenSiC grains, and (d) the weakened connects among different SiC grains.

Fig. 6 Quantitative analysis of surface morphology of the processed RS-SiC sample in PCVM by AFM:(a) surface morphology and (b) profile of the cross-sectional line.




characters as the amorphous silicon. What is more, sizes ofthe SiC grains were shrunken, as shown in Fig. 5(c), becausethe material removal in PCVM was conducted from theexternal to the internal on a SiC grain. Furthermore, theconnects among different SiC grains were weakened, andthere were some almost dissociative SiC grains, as shown inFigs. 5(c) and 5(d), because the chemical reaction rate in theboundary region was larger than that in the central area.Finally, these bumpy structures on the processed RS-SiCcould decrease the surface hardness, and the contactingareas between the polishing particle and the abraded samplecould be increased. All these features were propitious to

improve the surface quality and the MRR in the furtherPAP process.

Quantitative analysis of surface morphology of the proc-essed RS-SiC sample in PCVM was obtained by AFM, asshown in Fig. 6. Surfaces of the processed SiC grains inRS-SiC substrate almost remained in a plane, which could bejudged from the profile of the cross-sectional line shown inFig. 6(b). What is more, there were many holes formed atthe initial Si regions, and depths of the holes varied from20 to 60 nm, which further validated the characteristics inPCVM of RS-SiC sample.

Fig. 7 Illustrations of the initial surface, processed area in PCVM, and processed region in PAP.

Table 1 Experimental parameters in processing RS-SiC by PCVM.

Experimental parameters Values

RF power supply 100 W

Gap 500 μm

Heþ CF4 þO2 750, 5, 5 ml∕min, respectively

Scan mode and scan speed Raster scan, 200 mm∕min

Feed pitch 1 mm

Table 2 The summarized experimental parameters in processingRS-SiC by PAP.

Experimental parameters Values

RF power supply 32 W

Process gas and its flow rate Heþ H2O, 1.5 L∕min

Load and pressure 10 g, 10.9 kPa

Grinding stone and its size Ceria (CeO2), square (3 mm × 3 mm)

Polishing head and its size Aluminum (Al), round (Φ12 mm)

Eccentricity 9 mm

Polishing time 40 min

Fig. 8 Schematic diagram of the PAP process with the parameterscorresponding to Table 2.

Table 3 The summarized collocations of autorotation speed andrevolution speed in PAP.

ω1

ω2

75 rpm 234 rpm 386 rpm 544 rpm 715 rpm

70 rpm Group 1 — Group 7 — —

100 rpm Group 2 Group 4 — — —

130 rpm — Group 5 — Group 9 —

160 rpm — Group 6 — — Group 11

200 rpm Group 3 — Group 8 Group 10 —




Fig. 9 Evaluations of surface qualities of the processed RS-SiC in PAP corresponding to the differentgroups: (a) ω1 ¼ 70 and ω2 ¼ 75, (b) ω1 ¼ 100 andω2 ¼ 75, (c)ω1 ¼ 200 andω2 ¼ 75, (d)ω1 ¼ 100 andω2 ¼ 234, (e) ω1 ¼ 130 and ω2 ¼ 234, (f) ω1 ¼ 160 and ω2 ¼ 234, (g) ω1 ¼ 70 and ω2 ¼ 386,(h) ω1 ¼ 200 and ω2 ¼ 386, (i) ω1 ¼ 130 and ω2 ¼ 544, (j) ω1 ¼ 200 and ω2 ¼ 544, (k) ω1 ¼ 160and ω2 ¼ 715, and (l) the summarized data.




From research on the MRR, surface quality, and surfacemorphology of the processed RS-SiC sample in PCVM,it could be concluded that PCVM could be treated as anefficient method for the rapid figuring of RS-SiC, and thesurface quality should be further improved.

3.2 Reaction-Sintered Silicon Carbide SampleProcessed by Plasma-Assisted Polishing

The PCVM processed RS-SiC sample was further processedby PAP to improve the surface quality. The investigated RS-SiC sample was shown in Fig. 7. The process parameters inPCVM were summarized in Table 1. The size of the RS-SiCsample was 30 mm × 30 mm. The size of the processed areain PCVM was 20 mm × 20 mm, which was realized by con-trolling the apparatus in Fig. 1(a) in the raster scanning modewith the scan speed of 200 mm∕min and feed pitch of 1 mm.

The parameters in processing RS-SiC by the PAP weresummarized in Table 2. The schematic diagram of thePAP process was shown in Fig. 8. The autorotation speedω1 and revolution speed ω2 were adjusted to compare themachining results when other parameters were confirmed.

The generation rate of the oxide layer was establishedwhen the RF power, process gases, and their flow rateswere confirmed. The MRR of the oxide layer could be con-trolled by adjusting the autorotation speed and revolutionspeed when the type of grinding stone (polishing particle)and its size, polishing head and its size, and eccentricitywere assigned. In the experimental apparatus of PAP asshown in Fig. 1(b), there were five levels for both the autor-otation motor and the revolution motor. The selected experi-ments were organized in 11 groups corresponding to thedifferent collocations of autorotation speed and revolutionspeed, as shown in Table 3, which aimed to seek the optimalprocessing parameters.

After PAP, the processed RS-SiC samples were observedby SWLI to evaluate the surface roughness, and the resultswere shown in Fig. 9. For the purpose of conveniently com-paring the surface quality, the obtained surface roughnessrms in Fig. 9 was summarized in Table 4. It could befound that the optimal collocation was group 2 (correspond-ing to ω1 ¼ 100 and ω2 ¼ 75), in which the obtained surfaceroughness rms was 2.186 nm, as shown in Fig. 9(b).Meanwhile, the worst result appeared in the group 11 (cor-responding to ω1 ¼ 160 and ω2 ¼ 715), in which theachieved surface roughness rms was 13.080 nm, as shownin Fig. 9(k). The experimental results indicated that surface

quality of the processed RS-SiC in PAP was obviously betterthan that of the surface obtained by PCVM [Fig. 3(b)] andslightly worse than that of the surface obtained by diamondlapping [Fig. 3(a)]. Meanwhile, there were no visible subsur-face damages or scratches on the processed sample in PAP,because the hardness of the ceria abrasive grain was smallerthan that of the RS-SiC substrate and almost equal to that ofthe oxide layer. Relative to the obtained ultrasmooth surface(roughness rms 0.629 nm) in the divided PAP (D-PAP) proc-ess,5 the achieved surface quality in the simultaneousPAP (S-PAP) could be improved by optimizing the processparameters. Meanwhile, the rate in plasma oxidation ofRS-SiC was low and the oxide layer prevented the formeroxidation of subsurface layer. What is more, in D-PAP,the oxidation and the polishing processes were divided,which indicated that the MRR in D-PAP was obviouslysmaller than that in S-PAP. Therefore, S-PAP was more suit-able for the fine finishing of the RS-SiC sample.

The surface quality of the processed RS-SiC sample inPAP was determined by matching the MRR of oxidelayer v1 with its generation rate v2. In this study, the plasmaoxidation rate v2 of the RS-SiC sample was confirmed by theexperimental parameters listed in Table 2, and the MRR ofthe oxide layer v1 could be controlled by adjusting the autor-otation speed ω1 and the revolution speed ω2. Although theactual values of v1 and v2 are difficult to obtain, they couldbe confirmed under certain experimental conditions. Ifv1 ¼ v2, the generation of oxide layer entirely matched itsremoval, which could obtain a relatively smooth surfaceroughness, as shown in Fig. 9(b), and the PAP of RS-SiCcould simultaneously obtain a high MRR and smooth surfacein this condition. When v1 < v2, the oxide layer could not beremoved completely and the polishing process was equal tothe polished silica (SiO2) component by ceria abrasion,which was difficult to obtain an ultrasmooth surface, asshown in Fig. 9(a). Otherwise if v1 > v2, after removal ofthe oxide layer, the ceria abrasive continued to polish theRS-SiC substrate. The ceria abrasive was seriously abraded,because the hardness of the ceria abrasive grain was dis-tinctly smaller than that of the RS-SiC. The wear degreeof the ceria abrasive grain was aggravated along with theincreasing of ω1 and ω2, which resulted in a worse surfacequality of the processed RS-SiC sample in PAP, as shown inFigs. 9(c)–9(k). The experimental results indicated thatparameters in PAP of RS-SiC should be strictly selected,and the optimal parameters simultaneously could obtainhigh MRR and smooth surface.

4 ConclusionsThe combination of PCVM and PAP was conducted to verifythe feasibility of these techniques for processing an RS-SiCsubstrate, which was aimed to improve the MRR in rapidfiguring and ameliorate the surface quality in fine finishing.The following conclusions were obtained in this study:

1. Through observation of the processed RS-SiCsample in PCVM by SWLI, the calculated peak-MRR and volume-MRR were 0.533 μm∕min and2.78 × 10−3 mm3∕min, respectively. Relative to theconventional techniques for machining RS-SiC sample,the MRR of RS-SiC in PCVM was considerable.Meanwhile, there were no visible subsurface damages,tiny scratches, or residual stresses on the processed

Table 4 The obtained surface roughness rms of the polished RS-SiCsample in each group.

ω1

ω2

75 rpm 234 rpm 386 rpm 544 rpm 715 rpm

70 rpm 3.059 nm — 5.785 nm — —

100 rpm 2.186 nm 6.203 nm — — —

130 rpm — 6.545 nm — 6.328 nm —

160 rpm — 7.246 nm — — 13.080 nm

200 rpm 6.670 nm — 8.546 nm 9.635 nm —




RS-SiC sample because of the noncontact chemicalmaterial removal mechanism in PCVM. The resultsproved that PCVM could be treated as an efficientrapid figuring technique for RS-SiC products.

2. By comparing surface roughnesses and morphologiesof the RS-SiC samples before and after PCVM,it could be concluded that the processed RS-SiCsurface was deteriorated with surface roughness rms382.116 nm, and the major reason was that themicro-MRR of Si grain was larger than that of theSiC grain. Although the surface of the processedRS-SiC in PCVM was rough, the decrease of hardnessand increase of contacting area were propitious inimproving MRR in further polishing.

3. Evaluations of surface qualities of the processed RS-SiC samples in PAP corresponding to different collo-cations of autorotation speed and revolution speedwere conducted. The optimal surface roughness rmsof the PAP processed RS-SiC was 2.186 nm, whichwas obviously better than that of the surface obtainedby PCVM and slightly worse than that of the surfaceobtained by diamond lapping. What is more, therewere no subsurface damages, scratches, or residualstresses on the processed sample, because hardnessof the ceria abrasive grain was smaller than that ofthe RS-SiC and almost equal to that of the oxide.

4. Surface quality of the processed RS-SiC in PAP wasdetermined by matching MRR of the oxide withits generation rate. There existed three possibilities:insufficient polishing, optimal polishing, and overpolishing. The results indicated that parameters inPAP should be strictly selected, and the optimalparameters could simultaneously obtain high MRRand smooth surface.

The combination of PCVM and PAP can be treated as atechnique for RS-SiC, which can improve the machininglevel of RS-SiC samples and promote the application ofRS-SiC products.

AcknowledgmentsThis work was supported by a grant from the NationalKey Research and Development Program (GrantNo. 2016YFC0802903), a grant from the National NaturalScience Foundation of China (Grant No. 51505498), anda grant from the Natural Science Foundation of JiangsuProvince (Grant No. BK20150714). The authors also expresstheir gratitude to the staffs and students of the ResearchCenter for Ultra-Precision Science and Technology, OsakaUniversity.

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Xinmin Shen received his BS, MS, and PhD degrees in mechanicalengineering from the National University of Defense Technology in2008, 2010, and 2014, respectively. He is a lecturer at the PLAUniversity of Science and Technology. From 2011 to 2013, he studiedin Osaka University as a special research student. He is the author ofmore than 20 papers. His current research focuses on the ultrapreci-sion machining of optical components.

Kazuya Yamamura received his PhD in engineering from OsakaUniversity in 2001. He is an associate professor at Osaka University.His research area is development of unconventional ultraprecisionmanufacturing process and its application, such as figuring, finishing,functionalization, utilizing reactive plasma, or electrochemical process.

Biographies for the other authors are not available.




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